1. Field of the Invention
This invention relates to a friction-type stepless speed change device for general purposes, and more particularly to a friction-type stepless speed change device which is suitable for use in various kinds of industrial machines, transportation machines and the like.
2. Description of the Related Art
A friction-type stepless speed change device generally includes driving and driven friction wheels typically in the form of cones, discs, rings, spherical members and the like, which are urged into contact with each other. The contact position between the two friction wheels can be displaced to adjust the rotating radii of the respective friction wheels and thereby change the rotational speed of the driven friction wheel in a stepless manner. Depending upon the mode of contact between the friction wheels, the friction-type stepless speed change device is often classified into two types, i.e. external contact type and internal contact type.
In relation to external contact type speed change devices, transmission between the friction wheels is achieved by substantially point-like contact between two normally convex surfaces, so that the contact area is of substantially elliptical shape due to the contact pressure. Thus, when the contact area is observed as a rotary body, the contact track corresponding to the pitch lines is formed of a wide, belt-like area. Consequently, positive and negative slips occur on the outer and inner sides of the pitch diameter, respectively, and such slips result in increase in the internal friction loss to lower the transmission efficiency.
On the other hand, in the case of internal contact type speed change device, the transmission between the friction wheels is accomplished by contact between a concave surface and a convex surface, and the contact area or contact track is of substantially crescent-shaped, and is longer and narrower than that of the external contact type speed change device. Consequently, the contact can be assumed a line contact, and it is thus possible to significantly reduce the difference in rotational speed between outer and inner sides of the pitch diameter at the contact portion, and thereby reduce the internal friction loss.
As can be appreciated from the above explanation, in view of transmission efficiency, the internal contact type is generally superior to the external contact type since it is desirable for the friction discs to have a contact track of as reduced a width as possible.
The inventors already proposed certain improvement in the stepless speed change device which effectively makes use of the internal contact, and which is disclosed in Japanese Patent Application Publication No. 1-37,623.
The stepless speed change device according to the previous proposal includes input and output shafts, driving friction wheels which are adjustable in eccentricity relative to the input shaft, an internal gear arranged concentric to and formed integrally with the driving friction wheels, and a pair of internal gears which are arranged concentric to and formed integrally with the input shaft, one of which is connected via an intermediate transmission gear to the first-mentioned internal gear which is integral with the driving friction wheels. The speed change device further includes a hollow cylindrical driven rotary unit provided rotatably about the axis of the input shaft, driven friction wheels provided on the inner circumference of the driven rotary unit so as to be rotatable therewith and adapted to engage with the driving friction wheels under pressure, and a planet carrier integrally provided on the driven rotary unit. Planet gears are rotatably supported on the planet carrier and meshed with the other of the pair of internal gears, and also with a sun gear formed integrally with the output shaft which is arranged concentric to the input shaft. For changing the eccentricity of the driving friction wheels, they are formed into an integral driving rotary unit and fitted on an eccentric cam assembly composed of an internal cam member which is eccentrically fitted on the input shaft, and an external cam member which, in turn, is eccentrically fitted on the internal eccentric cam, but maintained in a concentric relation with the input shaft.
In general, a friction-type stepless speed change device transmits torque by friction at a contact point between the friction wheels, so that there is a limitation in increasing the torque which can be transmitted. To this end, friction wheels of a large diameter are usually employed to increase the rotational radii at the contact point and thereby increase the transmission torque. As a result, not only the device as a whole tends to become heavy and bulky due to the provision of large-sized friction wheels or the like components, but also the device is expensive to manufacture due to the requirement of high accuracy for these components.
Furthermore, in the case of a speed change device wherein the driven friction wheels of large diameter are maintained in parallel with each other by limited support portions only, as in the device according to the above-mentioned previous proposal, the driven friction wheels are in point contact with the driving friction wheels only at a single point so that the driven friction wheels may be subjected to eccentric load. It is often difficult to maintain the two driven friction wheels in parallel with each other, and the driving and driven friction wheels may thus contact each other in undesirable condition, accompanied by reduced transmission torque, fluctuation in the transmission efficiency or difficulty in achieving a smooth operation for changing the eccentricity of the driving friction wheels.